Sometime they hire under 18 PA's, but it's very rare. Especially during the summer months. Because the competition for the very few jobs gets even greater in the summer when high school students are looking for work. Yes, you are hired for the whole project and not just your connivence. This is one of the reasons why so few producers will hire PA's under 18. By law they can't work more than 10 hours a day and the typical PA is first to arrive and the last to leave making it a 14 hour day, easy. Money isn't the issue. Legality is. A person under 18 cannot be held liable for anything. Most cars on a set are rented and 16/17 year old drivers cannot drive rented cars so the "runner" part is out. You might want to try student films for experience.